Corundum, for one (5)
I believe the answer is:
oxide
'corundum for one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oxide that I've seen before include "Eg, rust" , "Rust, for instance" , "Compound of oxygen and one other element" , "Chemical compound" , "Oxygenous compound" .)